Download Regulation of the Urinary Bladder Chapter 26

Regulation of the Urinary Bladder
Chapter 26
Anatomy
1. The urinary bladder is smooth muscle lined internally by transitional epithelium and externally
by the parietal peritoneum. Contraction of the smooth muscle empties the bladder.
2. The external urinary sphincter (skeletal muscle) controls movement of urine through the
urethra.
Innervation
1. The parasympathetic division stimulates urinary bladder contraction. The parasympathetic
division is responsible for controlling micturition (urination).
2. The sympathetic division inhibits bladder contraction and stimulates internal urinary sphincter
contraction. It does not play a role in normal micturition. The sympathetic division is active
during ejaculation, preventing retrograde ejaculation of semen into the urinary bladder. Females
do not have an internal urinary sphincter.
3. The external urinary sphincter is tonically contracted as a result of stimulation from the brain. It
can relax as a result of reflex activity or from voluntary motor control.
4. Higher centers in the pons and cerebral cortex can facilitate (EPSPs) or inhibit (IPSPs) the
reflex center in the spinal cord.
Micturition Reflex
1. Filling of the bladder stimulates stretch receptors in the urinary bladder. A parasympathetic
reflex causes contraction of the bladder.
2. If urine is pushed into the urethra, a second reflex is activated that inhibits the external urinary
sphincter.
3. Stimuli from the stretch receptors reach the brain and awareness of the need to urinate occurs.
4. To go or not to go, that is the question.
A. Urination occurs if the higher brain centers facilitate the micturition reflex and cause
voluntary relaxation of the external urinary sphincter.
B. Urination does not occur if the higher brain centers inhibit the micturition reflex and
increase stimulation of the external urinary sphincter.
C. In infants, the higher brain centers do not inhibit urination.
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Awareness of the
need to urinate
Stimulatory and
inhibitory signals to
the micturition reflex
center
Tonic stimulation
and voluntary
control of external
urinary sphincter
Illustration Will Be Provided in Lab
Dorsal root
Conus medullaris
(end of spinal cord)
Sensory neurons
detect stretch
Ventral root
Parasympathetic
neurons stimulate
the bladder to
contract
Pelvic nerves
(S2-S4)
Urinary
bladder
External urinary
sphincter
Urethra
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Somatic motor
neurons stimulate
external urinary
sphincter
Pudental nerves
(S2-S4)
Disorders of Micturition
1. Incontinence is the inability to voluntarily control the external urinary sphincter. It can result
from spinal cord injury, pregnancy, inadequate maturation (infants), and old age.
2. Urinary retention is an inability or difficulty in emptying the bladder. It can result from an
enlarged prostate gland or anesthesia (inhibits smooth muscle activity). It is not uncommon that
following surgery patients are catheterized until the bladder recovers.
Effect of Spinal Cord Injury on Micturition
1. Recall that injury to the spinal cord produces spinal shock - lack of facilitory impulses
(EPSPs) from the brain results in depression of reflex centers. Thus reflexes are lost below the
level of injury.
2. Often, the reflex centers "compensate" for the lack of facilitory impulses and become
functional. Sacral reflex involving micturition and defecation return a few weeks following
spinal cord injury.
☞ Predict the effect of the following:
1. Transection of the spinal cord above the conus medullaris. Assume the patient has recovered
from spinal shock. What effect does the injury have on the following conditions?
A. The awareness of the need to urinate and the ability to voluntarily control urination?
B. The ability of the urinary bladder to empty?
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2. Damage to the conus medullaris. What effect does the injury have on the following conditions?
A. The awareness of the need to urinate and the ability to voluntarily control urination?
B. The ability of the urinary bladder to empty?
3. Damage to the dorsal roots of the pelvic nerves (caused by syphilis or crushing injuries). What
effect does the injury have on the following conditions?
A. The awareness of the need to urinate and the ability to voluntarily control urination?
B. The ability of the urinary bladder to empty?
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4. Decrease inhibitory signals to the reflex centers in the conus medullaris. What is the effect on
the following conditions?
A. The awareness of the need to urinate and the ability to voluntarily control urination?
B. The ability of the urinary bladder to empty?
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